Dielectric, Impedance and Conductivity Spectroscopy

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Dielectric, Impedance and Spectroscopy

This Article will discuss what is Dielectric, Impedance and Conductivity Spectroscopy with the addition of discussing the Similarities and differences

Impedance Spectroscopy

Dielectric, Impedance or Conductivity Spectroscopy primarily involve the measurement of the complex impedance spectrum Z*(ω) of a sample material positioned between two or more electrodes, which can be either liquid or solid. The acquired spectrum is subsequently analyzed within the context of the following two research domains.

Dielectric Spectroscopy and Conductivity Spectroscopy
The primary focus lies in examining material properties, with efforts typically made to mitigate contributions from electrode effects. Fundamental electrical material characteristics such as the complex permittivity ε*(ω) or conductivity σ*(ω) spectra can be readily derived from the complex Impedance Z*(ω) using sample dimensions. If the sample electrodes are substituted with an inductive coil filled with sample material, the complex magnetic permeability spectra μ*(ω) can be determined. Apart from frequency, electrical material properties are influenced by various additional parameters, with temperature being the most significant. Factors such as time, DC superimposed bias, AC Voltage Amplitude, and pressure dependency are commonly explored. Furthermore, investigations into non-linear properties, utilizing field dependence and higher harmonics, yield supplementary insights in this context.
Electrochemical Impedance Spectroscopy EIS
In this context, Impedance spectra are commonly acquired under controlled DC voltage (potentiostatic) or current (galvanostatic) conditions. The emphasis is primarily placed on exploring the characteristics of electrode/material interfaces. Electrodes typically consist of metal, while the materials involved are frequently electrolytes or ion conductors.

Impedance Measurement procedure

The impedance measurement process uses the complex impedance function Z*(ω) to assess the electrical properties of devices or materials . This function is determined by the ratio of the voltage across two electrical connections of a sample object to the resultant current passing through these connections.

The underlying concept of impedance measurement(simplified using 2 front wires connection)

In an impedance measurement, an alternating current (AC) voltage, denoted as U and with a fixed frequency f = 1/T, is applied to the sample under examination. This voltage induces a current, denoted as I, within the sample at the same frequency. Additionally, there typically exists a phase difference between the current and voltage, characterized by the phase angle φ. This renders all measured values to be complex by nature

The ratio of U to I and the phase angle φ are dictated by the electrical properties of the sample. To simplify calculation and representation of the formulas, it is advantageous to employ complex notation.

The resulting complex Impedance can be noted as:

$Z^{*} (ω) = \frac{U^{*} (ω)}{I^{*} (ω)}$

Other complex representations can be derived from measured impedance:

$\begin{array}{llll} Capacity: \\ C^{*} (ω) \\ = \\ \frac{1}{j ω Z^{*} (ω)} \\ Inductance: & L^{*} (ω) & = & \frac{Z^{*} (ω)}{j ω} \\ Admittance: & Y^{*} (ω) & = & \frac{1}{Z^{*} (ω)} \end{array}$

The amplitude and phase relationships between the voltage and current of a sample during an impedance measurement

As there is an angle between U and I which is the phase angle φ, all of the measured / calculated quantities can be represented as complex notation as follows:

$X^{*} = X^{'} + j X^{″}$

where X represents the measured / calculated value

The chosen quantity for measurement relies on the sample type and the researcher’s preference. As samples are inherently non-ideal, a representation closely resembling the electrical behavior of the sample is typically selected. For instance, a predominantly capacitive sample is often represented in terms of C(ω). Regardless of the specific representation, it’s crucial to note that they all encapsulate the same information: the sample’s response current to an applied voltage. Other commonly used representations include ratios of real and imaginary parts of the complex data, such as the loss factor tan(δ).

\tan δ = \frac{1}{\tan ϕ} = - \frac{Z^{'} (ω)}{Z^{″} (ω)} = - \frac{C^{″} (ω)}{C^{'} (ω)}

Vector representation of Z(ω) in the complex plane

Using Eynocs Impedance Analyzers, Fundamental complex material parameters such as permittivity ε(ω), conductivity σ(ω), and permeability μ(ω) spectra can be accurately and automatically determined across a wide frequency range, from microhertz up to hundred megahertz (spanning 14 decades), for nearly all types of materials. Sample preparation requires minimal effort, and equipment costs are relatively low compared to other material analysis methods. But while the measurement principle remains consistent, researchers, materials, underlying theories, models, and equipment requirements may vary considerably.

This mixture of features renders all impedance spectroscopy types as powerful and invaluable tools, particularly given the significance of electrical material properties in both fundamental and application-oriented research. These methods find application across diverse scientific communities tackling various materials and problems.